Pine needles need to be able to shed water to avoid their pores becoming waterlogged when it rains, which can lead to disease. How this happens has previously been studied using idealized fibres with circular cross-sections, but pine needles have wedge-shaped cross-sections. Writing in Physics of Fluids, Amy Lebanoff and Andrew Dickerson report that a wedge shape is more effective at shedding water than a round one is. Understanding water-shedding in nature is also useful when designing hydrophobic surfaces.
Pines start with a single long cylindrical needle, which splits into multiple needles as the plant matures. This lengthwise slicing of the cylinder creates needles which are round along one edge and wedge-shaped along the other. Lebanoff and Dickerson dropped water onto the needles and recorded high-resolution videos of the process (pictured). They used this footage to calculate how far along the fibre the drop spreads, the impact force of the drop and, most importantly, the amount of water retained on the fibre.
The experiment was done by either dropping water onto the wedge-up side of the needle or onto the round-up side of the needle. For a fixed drop size, the wedge-up fibres are more effective at splitting the drop into smaller droplets than their round-up counterparts. The impact force felt by the wedge-up fibres is larger, but the amount of water retained on the fibre is much lower. From an evolutionary context, the trade-off between impact force and water shedding is important as high-impact water drops can also damage the needles. This study indicates that the importance of water-shedding dominates in this case. Pine species that have more needles per bud, and so smaller wedge-angles per needle, are more effective at splitting incoming drops and shedding water.
Lebanoff, A. & Dickerson, A. Drop impact onto pine needle fibers with non-circular cross section. Physics of Fluids 32, 092113 (2020)
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Anirban, A. Raindrops falling on pine needles. Nat Rev Phys 2, 600 (2020). https://doi.org/10.1038/s42254-020-00253-7